The invention relates to a telemetry device for tracking radar and to the radar system comprising such a device.
In telemetry systems for conventional tracking radar means the telemetry circuits making it possible to measure the distance from a target echo and to ensure the tracking of such an echo by a telemetry mark permitting the production of a tracking window can be constituted by digital circuits. Such circuits are shown in FIG. 1 and comprise an arithmetic unit 2 which calculates the true distance of the echo from distance error information .epsilon.d or the displacement between the position of the echo signal and the tracking window produced by a variation or deviation detector 1. This distance is expressed, for example, in the time lag of the telemetry mark compared with the transmission pulse of the radar, when the latter is a pulse radar. To this end the deviation detector is supplied with a telemetry mark or a reference tracking window by on the one hand the video frequency signal of radar S and on the other by a generator system 3.
The generator system of the telemetry mark generally comprises a digital register 31 and an add-subtract counter 32 initially loaded before the start of each recurrence by means of the digital register 31 to the value of the distance expressed in delay time calculated by the arithmetic unit 2. The add-subtract counter 32 is supplied from the synchronisation signal S.sub.y triggered by the radar transmission signal by a very stable clock 33, for example in frequency pulses Fo. When N pulses have been subtracted by the add-subtract counter 32 the zeroing of the counter supplies a pulse constituting the telemetry mark to the deviation detector 1. The telemetry mark has a lag of N/Fo compared with the transmission pulse representing the distance position of the echo, the distance tracking window being, for example, produced at the deviation detector 1 for sampling the video signal S.
However, such a device has the disadvantage of supplying a telemetry mark whose lag or position suffers from a quantification error equal to a distance element corresponding to a time interval between two clock pulses, i.e. an error equal to the value 1/Fo. This quantification error in devices using the distance tracking window takes the form of a position uncertainty which causes jitter. This position jitter is particularly prejudicial to the elimination of fixed echos of tracking radar systems indicating moving targets, called MTI radar systems, because it leads to noise, whose amplitude has a detrimental influence on the Doppler filtering.
One solution for the above problem involves increasing the clock frequency Fo in order to reduce the quantification error. For an elimination level of the fixed errors .tau.EEF the tolerable jitter is substantially given by the ratio .tau./.tau.EEF, in which .tau. is the duration of the pulse transmitted by the radar, which leads to the choice of a very high clock frequency Fo, for example Fo=200 MHz for 0.5 .mu.s and .tau.EEF=40 dB.
Thus, the methods for producing the telemetry mark system are very difficult to perform and are also very costly.